1. Technical Field
The present invention relates to a method and an apparatus of measuring a film thickness for measuring a film thickness of a thin film sample by using a charged particle beam as well as a method and an apparatus of fabricating a thin film sample for fabricating a thin film sample by using a focused ion beam.
2. Background Art
When a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM) is used for analyzing a specific portion of a semiconductor device or the like, the main stream is constituted by fabrication of a sample using a focused ion beam machining. In TEM or STEM observation, an image formed by irradiating an electron beam to a thin film sample having a thickness to a degree of capable of transmitting electrons and enlarging a transmitted electron beam is acquired and observed. In order to acquire a clear observation image by the method, it is important to accurately measure a film thickness of a sample machined into a thin film by a focused ion beam.
Further, various micromachining technologies have been developed in accordance with a miniaturization of a semiconductor process in recent years. Not only fabrication of a sample for TEM or STEM but also a technology of accurately measuring a film thickness of a thin sample as in a thin film is requested in fabricating a microstructure.
As a background art method of measuring a thickness of a thin film in forming a sample into a thin film by irradiating an ion beam, there is disclosed a method of irradiating an electron beam to a sample face of the thin film and monitoring a thickness of a thin film from a secondary electron detecting amount detected by a secondary electron detector (refer to, for example, JP-A-8-240518).
An explanation will be given as follows by using a sectional view of a sample showing a method of measuring a film thickness of a background art of FIG. 20. Numeral 28 designates a region of diverging incident electrons at inside of an observation region 27, numeral 6 designates secondary electrons generated from the observation region 27, numeral 8 designates a secondary electron detector for detecting the secondary electron 6, and the secondary electron detector 8 is connected with a monitor apparatus for quantitatively monitoring a film thickness of the observation region 27 from an intensity of the secondary electron 6. When the observation region 27 is formed by irradiating a focused ion beam 21b, an electron beam 2b is irradiated to the observation region 27. Although when the electron beam 2b is irradiated thereto, the secondary electrons 4 are generated, in a case in which the film thickness of the observation region 27 is thick, the secondary electrons 4 are not generated from a face opposed to an incident face. However, in accordance with forming the observation region 27 into a thin film by etching machining by the focused ion beam 21b, an amount of secondary electrons generated from a side opposed to a side of being irradiated with the electron beam is increased. Therefore, the film thickness of the observation region 27 is quantitatively monitored from an amount of detecting the secondary electrons by the secondary electron detector 8 and an end point of focused ion beam machining can be determined.
According to the method and the apparatus of measuring a film thickness for quantitatively monitoring the film thickness from the secondary electron detecting amount of the background art mentioned above, it is necessary to acquire a calibration data by previously using a standard thin film sample to establish a relationship between the film thickness of the thin film and the secondary electron amount. At that occasion, the secondary electron amount is changed by a current amount of an incident electron beam, and therefore, the beam current amount of the electron beam needs to stay the same when the calibration data is acquired by using a standard thin film sample and when a film thickness of a desired sample is measured.
However, the current amount of the electron beam is changed over time in view of a property of an electron source, and a variation in the current amount is unavoidable even when the current amount is controlled by an electron optical system. Therefore, according to the method and the apparatus of measuring the film thickness, there is a case in which the current amount of the electron beam irradiated to the thin film is varied and the electron beam current amount when the calibration data is acquired by using the standard sample and the electron beam current amount when the film thickness of the desired sample is measured differ from each other, and therefore, the film thickness cannot be measured accurately. Further, when the film thickness of the desired sample is measured, the current amount of the electron beam can be adjusted to be the same as that when the calibration data is acquired by using the standard thin film sample by adjusting the electron optical system by measuring the current amount of the electron beam. However, according to the method, there poses a problem that time is taken, particularly when film thicknesses of a plurality of samples are measured, the film thicknesses cannot be measured easily in a short period of time.
The invention intends to resolve the problem provided to the method and the apparatus of the background art and it is an object thereof to measure a film thickness accurately, in a short period of time, and easily even when a current amount of a charged particle beam of an electron beam or the like to be irradiated is varied.